Four forces act upon an aircraft in relation to straight-and level, unaccelerated flight.
These forces are thrust, lift,
weight, and drag.
Thrust is the forward force produced by the powerplant/ propeller. It opposes or overcomes the force of drag.
Drag is a rearward, retarding force and is caused by disruption of airflow by the wing, fuselage, and other
protruding objects. Drag opposes thrust and acts rearward parallel to the relative wind.
Weight is the combined load of the aircraft itself, the crew, the fuel, and the cargo or baggage. Weight pulls the
aircraft downward because of the force of gravity. It opposes lift and acts vertically downward through the
aircraft’s center of gravity (CG).
Lift opposes the downward force of weight, is produced by the dynamic effect of the air acting on the wing, and
acts perpendicular to the flight path through the wing’s center of lift (CL).
The longitudinal, or roll, axis extends through the aircraft from nose to tail, with the line passing through the CG.
The lateral or pitch axis extends across the aircraft on a line through the wing tips, again passing through the CG.
The vertical, or yaw, axis passes through the aircraft vertically, intersecting the CG.
CG is the specific point where the mass or weight of an aircraft may be said to center; that is, a point around
which, if the aircraft could be suspended or balanced, the aircraft would remain relatively level.
The position of the CG of an aircraft determines the stability of the aircraft in flight.
As the CG moves rearward (towards the tail), the aircraft becomes more and more dynamically unstable. In aircraft
with fuel tanks situated in front of the CG, it is important that the CG is set with the fuel tank empty. Otherwise, as
the fuel is used, the aircraft becomes unstable. The CG is computed during initial design and construction and is
further affected by the installation of onboard equipment, aircraft loading, and other factors.
Major Components
Most airplane structures include a fuselage, wings, an empennage, landing gear, and a powerplant.
Fuselage
The fuselage is the central body of an airplane and is designed to accommodate the crew, passengers, and cargo. It
also provides the structural connection for the wings and tail assembly.
The most popular types of fuselage structures used in today’s aircraft are the monocoque (French for “single
shell”) and semimonocoque.
Wings
Many high-wing airplanes have external braces, or wing struts that transmit the flight and landing loads through the
struts to the main fuselage structure. Since the wing struts are usually attached approximately halfway out on the wing,
this type of wing structure is called semi-cantilever. A few high-wing and most low-wing airplanes have a full
cantilever wing designed to carry the loads without external struts.
The principal structural parts of the wing are spars, ribs, and stringers. These are reinforced by trusses, I-beams,
tubing, or other devices, including the skin. The wing ribs determine the shape and thickness of the wing (airfoil).
In most modern airplanes, the fuel tanks are either an integral part of the wing’s structure or consist of flexible
containers mounted inside of the wing.
Attached to the rear, or trailing edges, of the wings are two types of control surfaces referred to as ailerons and
flaps.
Ailerons extend from about the midpoint of each wing outward toward the tip, and move in opposite directions
to create aerodynamic forces that cause the airplane to roll.
Flaps extend outward from the fuselage to near the midpoint of each wing. The flaps are normally flush with the
wing’s surface during cruising flight. When extended, the flaps move simultaneously downward to increase the
lifting force of the wing for takeoffs and landings.
